This is true of other eras, as well. There was no defining moment when teams went from 4-man to 5-man rotations. Plus you can get teams even now who will use a 6-man rotation sometimes. Plus teams IRL can shift from one style to another during a season, for a variety of reasons.
However, the AI in OOTP is not sophisticated enough figure out when to use one style or another, based on a given team's pitching staff, so the game uses simple rules for the AI in all cases. Just something we have to live with in playing a computer sim.
